In textile production, synthetic or man-made fibers are typically formed from a molten polymer material in a "spin-draw" process. The molten polymer, commonly nylon or polyester, is spun into filaments, five to five hundred of which are twined together to form a single fiber. The spun fiber is then drawn, altering the fiber's elasticity and tensile strength, and producing a continuous fiber with a desired final diameter. Natural fibers are typically spun from short lengths of natural material, such as wool or cotton, to form a staple fiber.
During the processing of the fiber, a "finish", typically a liquid emulsion, is applied to the fiber to lubricate its filaments and reduce static electricity buildup generated by the flow of the fiber through the processing machinery. Generally, the finish is water-based for nylons and oil-based for polyesters.
The industrial production of polymer fiber generally occurs at rates of 2,000 to 8,000 meters per minute, and can be as high as 6,000 meters per minute. Uniformity of the physical properties of the resulting fiber is critical to the performance and processing by the destined end user. Lack of fiber uniformity can result in costly production line shutdown, and/or product irregularities for end users, such as entanglement or breakage of the fiber during a weaving process and inconsistent fiber coloration.
During fiber production, several variables, such as temperature, roll surface, drawing speed, processing parameters and uneven application of the finish can affect the uniformity and properties of the fibers. Conventional measures of a fiber's properties are denier, the fiber density expressed as the weight in grams of a 9,000 meter length of fiber; and percent finish-on-yarn ("%FOY"), a measure of the amount of finish on the fiber; and interlacing. Interlacing nodes, also known as entanglement, are added to the fibers to provide a bond between the individual filaments that constitute each fiber. Generally, interlacing nodes are created regularly along the length of a fiber, typically every 5-20 cm, by exposing the fiber to a high velocity stream of air. Other fiber properties that are commonly measured include evenness, a measure of the variation of a fiber's denier, node distribution along the length of a fiber, and the size and formation of the nodes.
Currently, there is no reliable and cost-effective way to monitor the properties of fibers during production in real time as the fiber is being produced. As a result, fiber properties are measured after production. If the fiber properties fall outside accepted tolerances, substantial portions, or even whole runs, of production can be lost.
It is therefore preferable to determine fiber properties substantially in real-time as the fiber is being produced. Such a real time measurement system should be able to withstand shock, vibration, electromagnetic interference from equipment within a textile plant, and electrostatic charge build-up from contact with the passing fiber. The system should be generally insensitive to exposure to solvents used to clean the production line equipment, and splattering by finish as it is applied to the fiber.
In addition, if a real-time measurement system is to be adaptable to a variety of production environments, it should operate on a variety of fiber types. Further, as a fiber is composed of multiple filaments which can have different cross-sectional geometries, such as round, hollow, triangular and multi-lobed, the measurement system should also operate with a wide variety of fiber cross-sections.
Devices and analytical techniques exist for non-intrusive investigation of materials to deduce their physical properties. For example, dielectric sensors can measure the effects that the investigated material has on the capacitance of an imposed field. However, as is well known, such sensors exhibit problems with air gaps (U.S. Pat. No. 5,045,798, Hendrick and U.S. Pat. No. 5,095,278, Hendrick). Air gaps severely limit the sensors' ability to measure dielectric properties of a sample material as air and a vacuum have the lowest theoretically possible permittivity.
U.S. Pat. No. 4,706,014 to Fabbri and U.S. Pat. No. 5,394,096 to Meyer employ capacitive sensors to measure the diameter of a polymer fiber and denier, respectively. However, capacitive techniques can detect only relatively large variations in denier, can show extreme sensitivity to finish, and can be highly susceptible to fiber orientation and cross-section. Thus, such systems are not generally useful.
Electrostatic sensors are also known but also suffer from disadvantages in that they do not exhibit good sensitivity, are strongly affected by humidity and must be maintained at a specific distance from the material under investigation.
U.S. Pat. No. 5,394,340, which is owned by the assignee of the present invention and the contents of which are incorporated herein by reference, discloses a dielectric and capacitive sensor system for identifying and measuring substances, such as ice, on an aircraft wing. In this sensor system, sensor electrodes are driven to produce an electric field in a pattern which approximates a Bessel function. Current measurements are then taken from predetermined electrodes and analyzed to determine the nature and thickness of a layer of material overlying the sensor. While this sensor system is appropriate for identifying the presence or absence of layered materials on a surface, it is not capable of measuring the physical properties of a material moving at high speeds past the sensor.